(1) Field of the Invention
The present invention relates to an apparatus for spatially resolved temperature measurement according to the preamble of claim 1.
(2) Description of Related Art
An apparatus of the aforedescribed type is disclosed, for example, in EP 0 692 705 A1. Fiber-optic temperature measurement systems (Distributed Temperature Sensing—DTS) can employ optical effects in optical fibers for spatially resolved temperature measurements. For example, the effect caused by Raman scattering can be used. The radiation from a narrowband source of electromagnetic radiation (e.g., from a laser) is inelastically scattered in the fiber material. The ratio of the intensities of the scattered radiation with a wavelength shorter than the excitation wavelength (anti-Stokes scattered radiation) and of the scattered radiation with a wavelength longer than the excitation wavelength (Stokes scattered radiation) is temperature-dependent and can be used to determine the temperature. By using frequency-domain techniques (Optical Frequency Domain Reflectometry—OFDR), which are described in EP 0 692 705 A1 and EP 0 898 151 A2, or pulse techniques (Optical Time-Domain Reflectometry—OTDR), the temperature can be determined along the fiber with spatial resolution. Such a temperature measurement systems can be used, for example, for monitoring fires in tunnels and ducts, for monitoring power cables and pipelines, and in the oil and gas exploration.
A DTS device generally includes, in addition to the corresponding coupling optics, the following essential optical components:
a laser light source,
a spectral splitter for coupling the light from a laser light source into the optical fiber used for the measurement and for separating the Raman scattered light portions of the laser light backscattered from the optical fiber,
an optical fiber used for the measurement,
a spectral splitter for separating Stokes and anti-Stokes scattered light,
filters for the Stokes and the anti-Stokes scattered light,
detectors for the Stokes and the anti-Stokes scattered light.
Instead of two filters, changeable or interchangeable filters can also be used for the Stokes and the anti-Stokes scattered light. When using interchangeable filters, both channels are measured sequentially. This is disadvantageous for the measurement time, but may have cost advantages as well as advantages for the accuracy, because identical channels are used for both signals.
A DTS device can principally be constructed mostly as free space optics. However, fiber-optic setups are frequently employed for a number of practical reasons (efficiency, stability).
One problem with spatially resolved temperature measurements using optical fibers is the change of the polarization along the fiber. This occurs mostly, but not exclusively, with single mode fibers. The exciting radiation is typically polarized. Because the Raman scattering can also be polarized, the Raman scattered light portions returned from the fiber can also be polarized. The Raman scattered light is detected with spectral splitters, filters and possibly other polarization-dependent components. The result of the measurement can therefore depend on the polarization.
In multimode fibers, the different modes propagate with slightly different velocities, and the effects of the fiber on the polarization are also mode-dependent. A more or less homogeneous mixture of different polarization states is formed over longer distances. The problem associated with polarization effects in DTS measurements hence exists predominantly in single mode fibers and in measurements with multimode fibers having few modes or short lengths.
In the fiber, the polarization plane can be rotated or the polarization can be changed in other ways by effects, such as stress-induced birefringence. The measured signals then depend not only in the desired manner on the local temperature, but also on the local polarization at the measurement location or the change of the polarization on the path through the fiber. Even if the polarization effects affect the measured quantities only slightly, they can still have a significant effect on the determination of the temperature, possibly reaching, for example, several ° C. Such effects can therefore limit the temperature resolution of DTS devices. In particular in devices operating with single mode fibers, modulations on the temperature curves with an amplitude of several ° C. and a wavelength of several meters to several 10 m are observed. These modulations are caused by rotations of the polarization plane due to stress-induced birefringence in the fiber material.
The problem forming the basis for the present invention is to provide an apparatus of the aforedescribed type which is capable of attaining higher temperature resolution and/or spatial resolution.